Plans to privatise probation services are being pushed through to an "impossible timetable" as ministers battle to claw back millions of pounds, campaigners have claimed.

Union leaders have raised a number of concerns about Government plans to privatise 70% of the service, and claim that more criminals could reoffend while on probation or parole as the changes are made.

General secretary of probation union Napo Ian Lawrence said: "These plans are ill thought-through, are proceeding to an impossible timetable and they are dangerous. They will increase costs and will lead to communication breakdown between probation workers.

"In addition it is scandalous that these proposals are being pursued when every probation trust has consistently met or exceeded all their targets which were set by the Ministry of Justice."

Napo claims that its questions about the plans have been left unanswered by the Government, which is trying to get contracts signed in the autumn as it attempts to save millions.

The union has expressed concern that the period between submission and selection of successful bids is only two months for 10-year contracts.

There are hundreds of unfilled vacancies in the service, it says, and thousands of current staff are lodging grievances as probation is split between the private sector, taking medium and low-risk criminals, and the state looking at high risk.

An influential committee of MPs is set to look at plans to reform probation this week, and Lord Ramsbotham will call for a parliamentary debate and vote over the plans.

Adviser to Napo Harry Fletcher said: "These plans are being rushed through without proper parliamentary scrutiny. There has been no primary legislation and therefore no vote and no debate in Parliament.

"There is a total lack of accountability and transparency. The structure itself is flawed, with the state keeping high-risk offenders, and the subsequent risk of staff burn-out, while the private sector takes the rest and misses changes in offender behaviour, therefore putting the public at risk."

Probation trusts are set to cease operation on May 31, and t housands of workers are expected to stage a second 24-hour strike against the plans at the end of this month.

Napo believes there is a greater risk of offenders committing more serious crime while the changes are pushed through.

About 400 serious crimes are committed by people on probation or parole each year, and the union claims that this could rise as there will not be enough staff in the private sector to properly recognise risk, and the chance of reoffending rises when a criminal's supervising officer changes.

Napo has been running a campaign since May last year against Justice Secretary Chris Grayling's plans to overhaul the criminal rehabilitation sector.

Contracts are to be split across private community rehabilitation companies (CRCs) in 20 English regions and one Welsh region, while the National Probation Service (NPS), a new public sector organisation, will be formed to deal with the rehabilitation of 31,000 high-risk offenders each year.

Last month, the union said up to 1,000 probation officers are expected to appeal against jobs assigned to them under the proposed shake-up.

Justice Minister Jeremy Wright said: "More than half a million crimes are committed each year by those who have broken the law before - we urgently need to tackle these stubbornly high re-offending rates.

"Our reforms will finally address the glaring gap that sees 50,000 short sentenced prisoners released on to the streets each year with no support, free to go back to their criminal ways.

"We have always been clear that they will be rolled out in a controlled way, with robust testing at every stage, and significant progress has already been made towards this.

"These necessary reforms have been subject to extensive parliamentary scrutiny - including a Justice Select Committee hearing, an opposition day debate and the Offender Rehabilitation Bill currently progressing through Parliament."